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Freestanding Elements are thin elements in your model that are exposed on 4 or more sides.
Why is it Relevant? Similar to small overhangs, freestanding elements like pipes, arms, legs, posts, beams, airplane wings etc. are susceptible to breakage if they are too thin for the printing and post processing of the model.
For additional information on why a part may break, see the Small Overhangs post. In that post, large overhangs are mentioned. If the portion of your model in question is large compared its connection and support on the model, you will want to use the minimum thickness specified for Freestanding elements at the connection point and throughout the piece.
Small Overhangs are elements of a model that are connected along one edge and then extrude unsupported for a short distance.
Why is it Relevant? Small surfaces extending off the main body of a model are susceptible to breakage or failure for a number of reasons. Learn what minimum spec to use when modeling to avoide breakage.
3D printer requirements. Some 3D printers will print supports to ensure the quality and integrity of an overhang is maintained, some 3D printers will always have support material in the build to maintain overhangs, and some printers will be unable to support overhangs unless they are quite thick because there is no support.
Post Processing. Many 3D printed parts will require some sort of handling after they come out of the machine. Very thin protrusions are susceptible to breakage at this point so 3D print Service Bureaus will set a minimum thickness of extrusions or overhangs to reduce breakage costs. If you REALLY need to have thinner parts...
Surface Relief pertains to elements, both recessed and extruded, that you wish to show on your model at your chosen scale.
Why is it relevant? Knowing your chosen printer's tolerances for surface relief is necessary because you may need to Accentuate or over-accentuate particular details in order to have them show up as clearly as you would like. Of course this is determined by knowing the scale you will be printing at and the machine you will be printing on. If you are unsure of either variable, you can at least draft with the concept in mind.
An Example. If I want to print a surface with a physical texture map, scales or tiles or rocks or hair or even lettering, and I want the effect to show a crisp relief on the surface, I would take the minimum suggested value from the calculator and apply this to my model. So, a model printed at 1/16th scale (also known as 1"=16' or 1:192) and I want to print on a FDM printer, the calculator will show that the Surface Relief detail...